At the beginning stage of cotton aphids (Aphis gossypii Glover) invading the cotton field, large numbers of insects have accumulated in the marginal alfalfa (Medicago sativa L.) zone surrounding it. They are 6.94 times (excluded natural enemies) as many as the aphids in cotton field. Most of these insects areTherioaphid maculata (Buckton), and occur in fields 10–15 d earlier than cotton aphids. Consequently, abundant natural enemies have bred in alfalfa zone when cotton aphids increase rapidly in cotton fields in the middle of June in Xinhe County, Xinjiang Autonomous Region. The total number of the major natural enemies, including Coccinellids, Chrysopids and Syphid flies in alfalfa zone, is 13.65 times that in cotton field within the same acreage of fields. At this period, it is very effective to force the natural enemies from the alfalfa zone into the nearby cotton field to control the rapid increase of cotton aphid population by cutting the alfalfa stands.